1. Field of the Invention
The present invention relates to a thermo-responsive micro-capsule which can be applied for a heat-sensitive recording material, and a heat-sensitive recording material and a multicolor heat-sensitive recording material using the microcapsule.
2. Description of the Related Art
A heat-sensitive recording material in which a solid dispersion of an electron donative dye precursor is coated and dried, on a substrate and is widely used as a recording medium for a facsimile, a printer or the like. A recording method using an electron donative dye precursor is advantageous because raw materials are easily available and high color developing density and color developing speed are manifested. However, this method is disadvantageous because color easily develops due to storage conditions after recording, heat or adhesion of a solvent and the like. Accordingly, there are problems in storability and reliability of a recorded image, and, improvements therefor have been studied.
As a method for improving storability of a recorded image there is suggested a method in which storability of an image is enhanced by encapsulating an electron donative dye precursor and isolating the dye precursor from a color developing agent in a recording layer. By this method, a high color developing ability and image stability can be obtained.
As a heat-sensitive recording material other than the above-described material, a so-called diazo type heat-sensitive recording material is also studied using a diazonium salt compound. This diazonium salt compound reacts with a phenol derivative, a compound having an active methylene group (coupler) and the like to form a dye. However, the diazonium salt compound loses its activity by irradiation of light since the compound has also a photosensitivity simultaneously. By utilizing these characteristics, a diazonium compound is recently applied also to a heat-sensitive recording material, and a photo-fixing type heat-sensitive recording material by which an image is formed by allowing to react a diazo compound with a coupler by heating, followed by fixing by irradiation with light is suggested (K. Sato et al., "Bulletin of the Society of Image and Electron", vol. 11, (4), 290-296 (1982), and the like).
However, since a recording material using a diazonium salt compound has a high chemical activity, there is a drawback that the diazonium compound and a coupler gradually react with each other even at a low temperature, and consequently storage life (shelf life) thereof is short. As a means to solve this drawback, there is suggested a method in which a diazonium salt compound is encapsulated in a microcapsule to be isolated from a coupler, water and a basic compound (T. Usami et al., "Bulletin of the Society of Electrophotography", vol. 26, (2), 115-125 (1987)).
Further, as an application of a heat-sensitive recording material, a multi-color heat-sensitive recording material comes into notice. It has been said that reproduction of multi-color images by heat-sensitive recording as compared with an electrophotographic recording method or an ink jet recording method is difficult. However, it has already been known that a multi-color heat-sensitive recording material can be obtained by layering, on a substrate, two or more of heat-sensitive color forming layers containing, as main component, an electron donative dye precursor and a color developing agent, or a heat-sensitive color forming layer containing a diazonium salt compound and a coupler which reacts with the diazonium salt compound by heating to form a color.
In a multi-color heat-sensitive recording material, it is essential to precisely control heat developing characteristics of a microcapsule to achieve an excellent color reproduction.
Conventionally, in order to encapsulate an electron donative dye precursor and a diazonium salt compound in a microcapsule, these compounds are generally dissolved in an organic solvent (oil phase) and this phase is added to an aqueous solution of a water-soluble polymer (water phase) to be emulsified and dispersed. In this procedure, by adding a monomer or a prepolymer which becomes a wall material to the organic solvent phase or the water phase previously, a polymer wall can be formed on the interface between the organic solvent phase and the water phase to make a microcapsule. This method is described in detail in "Microcapsule" (A. Kondo, Nikkan Kogyo Shimbun (Daily Industrial Newspaper K.K.) (1970)) and "Microcapsule" (Y. Kondo et al., Sankyo Publishing K.K. (1977)). As a material for the microcapsule wall to be formed, various materials can be used such as gelatin, alginate, cellulose, polyurea, polyurethane, melamine resin, nylon and the like. Polyurea and urethane resins are suitable for designing a heat-sensitive recording material since the glass transition temperature thereof is from room temperature to about 200.degree. C. and the resulting capsule wall shows thermo-responsibility.
In the case of a microcapsule having a polyurethane or polyurea wall, for producing the microcapsule, there is conventionally known the following method. First, a diazonium salt and an electron donative dye precursor are dissolved in an organic solvent. To this solution is added a multifunctional isocyanate compound, and the resulting organic phase solution is emulsified in a water-soluble polymer aqueous solution. Then, a catalyst which promotes polymerization reaction is added to the water phase or the temperature of the emulsion is raised to polymerize the multifunctional isocyanate compound with a compound having an active hydrogen such as water and the like, to form a capsule wall.
As the multifunctional isocyanate compound which is a forming material for a polyurea or polyurethane wall, an adduct of 2,4-tolylene diisocyanate with trimethylolpropane and an adduct of xylene diisocyanate with trimethylolpropane are, for example, mainly used (Japanese Patent Application Laid-Open (JP-A) Nos. 62-212190, and 4-261893).
However, even the above-described capsule wall made of polyurea or polyurethane using the multifunctional isocyanate compound does not sufficiently improve the above-described short shelf life when the diazonium salt compound is used. Namely, if the heat-sensitive recording material which does not have sufficiently long shelf life is exposed, for example, to high temperature and high humid conditions during period between the production and use thereof, color formation of the ground called "fog" is manifested, and visual identification ability of the resulting printed image deteriorates.
To solve such a problem, for example, the wall of a microcapsule is thickened, and the like. However, when such a method is used, color developing sensitivity in thermal printing lowers.
Therefore, further improvements on shelf life while maintaining a high color developing ability are extremely difficult.
To solve such a problem, there is known, f or example, a method in which a part of a multifunctional isocyanate compound is allowed to react with a monoalcohol compound previously before use, as described in Japanese Patent Application Laid-Open (JP-A) No. 5-317694. However, since specific monoalcohol used in this case is a compound having 2 to 9 carbon atoms, when ratio of use of the alcohol is raised, though sensitivity thereof increases, fog also increase. On the contrary, when ratio of use of the alcohol is reduced, though fog can be prevented, effect of increase in sensitivity is insufficient.
Further, the above-described multicolor heat-sensitive recording material comprises heat-sensitive recording layers for forming cyan, magenta and yellow colors, respectively, and each layer is heated at different temperatures for printing. Therefore, excellent thermo-responsiveness is further required as compared with the heat-sensitive recording layer of a usual heat-sensitive recording material. It cannot be said that the wall of the above-described conventional polyurea and polyurethane capsule wall fully satisfies these requirements.
A heat-sensitizer for improving heat sensitivity can further be added into the heat-sensitive color forming layer of a heat-sensitive recording material. As the heat sensitizer, p-toluenesulfonamide described in Japanese Patent Application Publication (JP-B) No.6-55546 and the like are known to exhibit excellent performance. However, as a compound having further excellent performance, there is listed an arylsulfonamide compound having a specific substituent described in Japanese Patent Application Laid-Open (JP-A) No. 9-39389. In a multi-color heat-sensitive recording material, it is necessary to emulsify the above-described arylsulfonamide compound before use to reduce haze in the above-described heat-sensitive color forming layer. Methods for emulsification are not particularly limited, and conventional known methods can be used. Specifically, the above-described arylsulfonamide compound is dissolved in an organic solvent which is slightly soluble or insoluble in water, the solution is mixed with a water phase containing a surfactant and/or water-soluble polymer as a protective colloid, and is stirred to prepare a emulsified dispersion. This emulsified dispersion is described in detail in Japanese Patent Application Laid-Open (JP-A) No. 2-141279.
However, since the heat sensitizer is usually a crystalline substance, an emulsified material containing the sensitizer sometimes causes problem that crystals of the sensitizer are deposited with a lapse of long period of time and the like. Accordingly, it has been desired that a microcapsule which manifests sufficient heat sensitivity without using the above-described heat sensitizer or using a small amount of the sensitizer is developed.